For many years, optical modulators have been made out of electro-optic material, such as lithium niobate. Optical waveguides are formed within the electro-optic material, with metal contact regions disposed on the surface of each waveguide arm. A continuous wave (CW) optical signal is launched into the waveguide, and an electrical data signal input is applied as an input to the metal contact regions. The applied electrical signal modifies the refractive index of the waveguide region underneath the contact, thus changing the speed of propagation along the waveguide. By applying the voltage(s) that produce a π phase shift between the two arms, a nonlinear (digital) Mach-Zehnder modulator is formed.
Although this type of external modulator has proven extremely useful, there is an increasing desire to form various optical components, subsystems and systems on silicon-based platforms. It is further desirable to integrate the various electronic components associated with such systems (for example, the input electrical data drive circuit for an electro-optic modulator) with the optical components on the same silicon substrate. Clearly, the use of lithium niobate-based optical devices in such a situation is not an option. Various other conventional electro-optic devices are similarly of a material (such as III-V compounds) that are not directly compatible with a silicon platform. Moreover, it is well-known that any of these field-based devices have inherent performance limitations at data rates exceeding, for example, 1 GB/s. In particular, lithium niobate-based arrangements need to be modeled as traveling wave structures, with relatively complex electrical drive structures required to attempt to have the device operate at the requisite speed.
A significant advance has been made in the ability to provide optical modulation in a silicon-based platform, as disclosed in U.S. Pat. No. 6,845,198 issued to R. K. Montgomery et al. on Jan. 18, 2005, assigned to the assignee of this application and incorporated herein by reference. FIG. 1 illustrates one exemplary arrangement of a silicon-based modulator device as disclosed in the Montgomery et al. patent. In this case, a silicon-based optical modulator 1 comprises a doped silicon layer 2 (typically, polysilicon) disposed in an overlapped arrangement with an oppositely-doped portion of a sub-micron thick silicon surface layer 3 (often referred to in the art as an SOI layer). SOI layer 3 is shown as the surface layer of a conventional silicon-on-insulator (SOI) structure 4, which further includes a silicon substrate 5 and a buried oxide layer 6. Importantly, a relatively thin dielectric layer 7 (such as, for example, silicon dioxide, silicon nitride, potassium oxide, bismuth oxide, hafnium oxide, or other high-dielectric-constant electrical insulating material) is disposed along the overlapped region between SOI layer 3 and doped polysilicon layer 2. The overlapped area defined by polysilicon layer 2, dielectric 7 and SOI layer 3 defines the “active region” of optical modulator 1. In one embodiment, polysilicon layer 2 may be p-doped and SOI layer 3 may be n-doped; the complementary doping arrangement (i.e., n-doped polysilicon layer 2 and p-doped SOI layer 3) may also be utilized.
FIG. 2 is an enlarged view of the active region of modulator 1, illustrating the optical intensity associated with a signal propagating through the structure (in a direction perpendicular to the paper) and also illustrating the width W of the overlap between polysilicon layer 2 and SOI layer 3. In operation, free carriers will accumulate and deplete on either side of dielectric layer 7 as a function of the voltages (i.e., the electrical data input signals) applied to doped polysilicon layer 2 (VREF2) and SOI layer 3 (VREF3). The modulation of the free carrier concentration results in changing the effective refractive index in the active region, thus introducing phase modulation of an optical signal propagating along a waveguide defined by the active region. In the diagram of FIG. 2, the optical signal will propagate along the y-axis, in the direction perpendicular to the paper.
FIG. 3 illustrates an exemplary prior art silicon-based Mach-Zehnder interferometer (MZI) 10 that is configured to utilize silicon-based modulating devices 1 as described above. As shown, prior art MZI 10 comprises an input waveguide section 12 and an output waveguide section 14. A pair of waveguiding modulator arms 16 and 18 are shown, where in this example waveguide arm 16 is formed to include a modulating device 1 as described above.
In operation, an incoming continuous wave (CW) light signal from a laser source (not shown) is coupled into input waveguide section 12. The CW signal is thereafter split to propagate along waveguide arms 16 and 18. The application of an electrical drive signal to modulator 1 along arm 16 will provide the desired phase shift to modulate the optical signal, forming a modulated optical output signal along output waveguide 14. A pair of electrodes 20 are illustrated in association with modulator 1 and used to provide the electrical drive signals (VREF2, VREF3). A similar modulating device may be disposed along waveguiding arm 18 to likewise introduce a phase delay onto the propagating optical signal. When operating in the digital domain, the electrodes may be turned “on” when desiring to transmit a logical “1” and then turned “off” to transmit a logical “0”.
To the first order, the output power of a conventional modulator as shown above is given by the equation:Pout=Pin/2(1+cos Δφ),where Pout is the output power from the modulator, P0 is the input power, and Δφ is the net optical phase difference between the two arms (e.g., arms 16 and 18 of modulator 10 of FIG. 3). As a result, the optical output power level is controlled by changing the value of the net phase shift φ between the two arms. FIG. 4 is a plot of this relationship, illustrating the output power as a function of phase shift between the two arms (a “1” output associated with maximum output power Pout and a “0” output associated with minimum output power Pout). That is, a differential phase shift between the two arms of the modulator provides either constructive interference (e.g., “1”) or destructive interference (e.g., “0”). Although not shown or described, it is to be understood that in implementation such a modulator may utilize a DC section to optically balance the arms and set the operating point at a desired location along the transfer curve shown in FIG. 4.
There have also been advances in the art of silicon-based optical modulators in terms of utilizing advanced signaling formats. See, for example, U.S. Pat. No. 7,483,597 issued to K. Shastri et al. on Jan. 27, 2009, assigned to the assignee of this application and herein incorporated by reference. As disclosed therein, a multi-bit electrical input data is used and the modulator itself is configured to include at least one modulator arm comprising multiple sections of different lengths, with the total length being equal to one π phase shift. One such exemplary modulator 25 is shown in FIG. 5. Each separate section is driven with an digital logic “1” or a digital logic “0”, that is, digitally driven to either be “on” or “off”, creating the multi-level modulation.
It is known that each modulator section can be optimized in terms of nominal length to provide nearly equal power levels in absolute value, regardless of the position of the section along the modulator arm (i.e., its “position” relative to the cosine-based power curve). Referring again to the transfer function curve of FIG. 4, it is clear that longer length modulation sections are needed to operate at the peak and valley of the cosine curve and provide the same output power change as sections associated with the “steeper”, central area of the transfer curve.
While the arrangement disclosed in Shastri et al. is useful for allowing a multi-bit data signal to drive a silicon-based optical modulator, it has been recognized that the free-carrier dispersion effect utilized for optical phase modulation in silicon exhibits a nonlinear phase modulation response, while also exhibiting attenuation that is proportional to the amount of phase modulation. FIG. 6(a) is a plot of the nonlinear phase modulation response versus applied voltage and FIG. 6(b) is a plot of attenuation of a silicon-based optical modulator as a function of applied voltage for the prior art device of FIG. 5. As shown in FIG. 6(a), the phase modulation is nonlinear for applied voltages less than about one volt, where the attenuation as shown in FIG. 6(b) increases as the applied voltage increases, reaching a value approaching 3 dB/mm for an applied voltage of 2V and an operating wavelength of 1550 nm.
Thus, a need remains in the art for a silicon-based optical modulator that recognizes and addresses the nonlinearity and attenuation problems associated with the free-carrier dispersion effect in these silicon devices.